The Rogers Pass spring raptor migration study in west-central Montana was an effort to monitor long-term trends in populations of raptors, primarily Golden Eagles, using this northern portion of the Rocky Mountain Flyway. Veteran raptor observers Fred and Cathy Tilly began studying the eagle migration through Rogers Pass in the late 1980's and conducted the first extended count in the area in 1988. Fred conducted a second extended count in 1990 and in 1992, working under HWI sponsorship and with wife Cathy as the second observer, began conducting annual counts during the month of March. Through 2002, they had recorded 14 species of migratory raptors at the site, with counts typically ranging between 1,200 and 2,200 migrants per season. Golden and Bald Eagles typically comprised 93-96% of the count, and the month-long effort was designed to sample primarily the adult Golden Eagle migration.
The interface of the Rocky Mountains and Great Plains represents an obvious migratory corridor for raptors, especially Golden Eagles. Prevailing westerly winds along the Rocky Mountain foothills create reliable updrafts for migrating raptors in a nearly continuous line extending from north-central Mexico through New Mexico, Colorado, Wyoming, Montana, and western Canada. The attractiveness of this route for spring, northbound migrants (especially eagles) is enhanced by the abundance of Columbian ground squirrels and black-tailed prairie dogs newly emerged from winter burrows, carrion from spring calving, and migrating waterfowl. All are available on the plains in close proximity to the Rocky Mountain foothills.
From 1993 through 2002, the Rogers Pass count occurred on the private Blacktail Ranch owned by Tag Rittel and Sandra Renner. For this reason, the site was not well-suited to intensive visitation and educational activities; however, prearranged visits by small numbers of people were always possible. Because recruiting replacements for the Tilly's on this project was very difficult, and because funding for the project was always limited, HWI has since decided to curtail its involvement in this project in favor of other priorities.